The edible mushroom Agaricus bisporus (Lange) Imbach var. bisporus, a basidiomycete fungus, is widely cultivated around the world. In Europe and North America, it is the most widely cultivated mushroom species. The value of the annual Agaricus bisporus mushroom crop in the United States was about $920,000,000 in 2003-2004, according to the National Agricultural Statistics Service, Agricultural Statistics Board, U.S. Department of Agriculture (Aug. 16, 2004). More than 90 percent of the Agaricus mushrooms cultivated in the United States, Europe, and elsewhere have a white pileus color, in accordance with consumer preferences.
Approximately 25 years ago, the first two white hybrid strains of A. bisporus, developed by a laboratory at Horst, the Netherlands, were introduced into commercial cultivation. These two “Horst” strains, called U1 and U3, are closely related crosses between two pre-existing white cultivated strains, as per M. Imbernon et al., Mycologia, 88(5), 749-761 (1996), herein incorporated by reference. The U1 and U3 strains, while still cultivated at present, are additionally thought to be the direct progenitors of all other white A. bisporus mushrooms currently cultivated in most regions of the world. Commercial mushroom strains developed from U1 and U3, such as A15 and S130, are all either clones or quasi-clones of U1 or U3, being developed either by clonal vegetative propagation or from spores which retain the great majority of the parental genotype, as shown by R. W. Kerrigan et al. in Genetics, 133, 225-236 (1993), herein incorporated by reference. A group of strains developed either by cloning or by spore propagation, or both, from a single progenitor (as opposed to outcrossing between two different progenitors) is called a lineage group. Except for minor acquired genetic differences all white strains developed within the Horst U1 lineage group and Horst U3 lineage group share a single basic genotype with the original U1 or U3 strains, respectively (which are themselves very similar, due to their close relationship). For these reasons, and the fact that the Horst U3 lineage group is presently cultivated to a much smaller extent than the Horst U1 lineage group, modern white Agaricus mushroom cultivation is effectively a monoculture. Hence, for purposes of this disclosure, all of these cultivar strains will be described hereinafter as the “Horst U1/U3 lineage group” where both the Horst U1 lineage group and Horst U3 lineage group are implied.
Currently, one of the most commercially successful representatives of the Horst U1/U3 lineage group is a strain designated A15 by the assignee of record. That strain, specifically, is from the Horst U1 lineage group.
The introduction of new varieties of white Agaricus bisporus mushrooms into commercial culture has been impeded by three difficulties. First, cross-breeding strains of Agaricus bisporus var. bisporus can be difficult and cumbersome. U.S. Pat. No. 5,304,721 sets forth many of the problems associated with cross-breeding. Second, experience indicates that most wild germ plasm resources for this species exhibit various traits that would be unacceptable in the marketplace. Third, most of these germ plasm resources incorporate alleles that give rise to brown mushrooms, which are in less demand by consumers than are white mushrooms. Color is predominately determined by alleles at the Ppc-1 locus; see P. Callac et al., Fungal Genetics and Biology, 23(2): 181-188 (1996), herein incorporated by reference. Alleles providing the white color trait are rare to relatively uncommon in most wild populations of A. bisporus. Of approximately 150 wild Agaricus bisporus mushroom strains collected in coastal California, only 2 were white, while the rest were brown, as seen in, inter alia, R. W. Kerrigan and I. K. Ross, Mycologia, 81(3):433-443 (1989), R. W. Kerrigan et al., Molecular Ecology, 7:35-45 (1999), herein incorporated by reference.
The difficult nature of breeding a commercially successful hybrid variety of A. bisporus is illustrated by the fact that very few patent applications for novel hybrid Agaricus bisporus strains have been filed in the United States; of these, only one (i.e., assignee of record's brown hybrid strain X618, marketed as S600) enjoyed even moderate commercial success. It is believed that no novel hybrid white mushrooms other than U1 and U3 have heretofore ever been successfully introduced into commerce in the United States.
There is a wide range of potential benefits to introducing greater diversity of strains into commercial cultivation. Novel strains may exhibit novel patterns of nutritional resource utilization, different responses to environmental manipulation, precocity or different developmental schedules, and novel aesthetic and culinary properties for the consumer. Examples of traits favored by the consumer could include a smooth, bright white cap surface, a more attractive shape (i.e., more round) or a greater development of pileus tissue (i.e., greater “meatiness” or thickness). Some of these benefits may become apparent only after years of cultivation and marketing experience, for example, if a novel crop pathogen emerges.
New strains may offer improved resistance to known and emerging diseases of the crop. In particular, they are potentially more resistant to infection by established viral diseases that are transmitted by anastomosis (i.e., the fusion of fungal cells, called hyphae). Empirically, it is known that, for two individual heterokaryotic strains of basidiomycete fungi, anastomosis is often difficult and perhaps even impossible, and generally unsuccessful. This condition is called “vegetative incompatibility.” A more detailed description of anastomosis and of some viral diseases to which basidiomycete fungi are susceptible can be found in A. S. M. Sonnenberg et al., Mushroom Science 14, 587-594 (1995), incorporated herein by reference.
Instances of incompatibility between strains of the mushroom Agaricus bisporus, and in other species of basidiomycete fungi, have been noted for many years. However, there is no real understanding of how this self/non-self recognition system works. It is not known for Agaricus bisporus how many genetic loci are involved, where they occur on the chromosomes, how many alleles are present at any locus, or what the specific effects of any allele or locus might be. Consequently it is not possible to predict what the compatibility phenotype of any new hybrid might be.
As noted above and since the 1980s, the only strains of white Agaricus bisporus mushrooms now grown commercially in North America and Europe, and in most other parts of the world, are strains derived from the Horst U1/U3 lineage group, and most particularly, the ‘Horst U1’ group. All share very similar genetic identities and all are compatible within and among the group, a situation known to agronomists as a ‘monoculture’. The industry has essentially standardized on this inter-compatible group of strains, and no widely-accepted alternative white strains have been introduced since 1980.
Prior to the 1980s, when a more diverse set of commercial strains was in use, crop rotation was sometimes adopted in response to the establishment of virus disease at commercial facilities. However the current situation of monoculture has, in recent decades, made it impossible to implement a scheme of crop rotation to allow for an interruption of strain identity and compatibility at production facilities. This situation of monoculture allows pathogens to become more perfectly adapted to the host strain, and to establish reservoirs of pre-adapted infectious material, in production facilities, and, in the case of obligate intracellular pathogens including the dsRNA viruses of Agaricus, also allows them to pass freely from established infection reservoirs into new strain-compatible crops.
In modern mushroom production facilities, a crop of mushrooms may typically occupy production space for 46-57 days, between the planting of spawn in the compost and the emptying and cleaning of the facility to prepare for the initiation of the next crop. During that crop cycle, three ‘flushes’ of mushrooms are normally harvested at about weekly intervals. In some cases, the scheduling of new crop production cycles may lead to the disposal of the prior crop before three full flushes of mushrooms can be obtained. There are three potential opportunities for accelerating this crop cycle: (1) during the spawn run interval, typically of 13-16 days duration, (2) during the case run interval, typically of 15-19 days duration, and (3) during the flushing/harvest periods, typically of 18-22 days duration. There are several economic benefits that derive from a shorter crop cycle: (a) higher utilization of physical plant, and distribution of overhead costs over more crops, (b) ability to routinely complete harvest of the third flush, increasing productivity, (c) opportunity to harvest more of the crop earlier with respect to disease pressure, which impacts crop quality and value, and (d) additional scheduling flexibility represented by multiple strain-schedule options.
In some markets, mechanical harvesting is preferred due to the higher cost of human labor. In order for a mushroom crop to be suitable for mechanical harvesting, uniform development and a sufficiently long stem are necessary and desirable.